Comptometers - Testing

If you own or are selling a Comptometer, a question that almost always arises is "does it operate correctly and will it give correct answers?" So here are a few relatively simple tests you can perform to test all machine operations.

But first a few general rules...

Clear the machine (to zeros) before each test.
Give each key a firm and full downward stroke.
Try to strike each key in the center of the keytop.
If a key strongly resists your stroke DO NOT FORCE IT!

If you have a machine with SN 100,000 or higher, the stubby red key next to the 9-key in the first column (the "Controlled Key") should release the keyboard when it is locked due to an incomplete stroke.
"

The 9s test

Enter all 9s on the keyboard and then add "1" to it. The machine should clear (to zeros) except for the high order answer dial which should have a "1" in it. This tells us that the basic carry mechanism is working, altho other tests will be needed to guarantee this.

The "all keys" test

Start by hitting any key and proceed to hit each key on the keyboard once and only once. While this will work in ANY sequence, you'll find it easier to start with a key in the corner of the keyboard and simply keep moving to the adjacent key row by row or column by column.

Your answer should be 499...995. Now hit the 5-key in the (¢) column and the register should read 500...000. To guarantee the leftmost dial operation, hit the 5-key in the leftmost column ten times and the register should now be clear.

The "all 5s duplex" test
(Note: not applicable to "Woodies")

Place all four fingers of each hand over the 5-keys and press them simultaneously, twice. You should see ..111111110 in the register. If you have a 10- or 12-column machine, clear it and do the same thing from the leftmost column (you get the idea).

Testing the "carry inhibit" tabs

With the left hand, press in on the units (¢) tab and hit the 5-key twice. If the "dimes" column to the left retains its zero, that tab works correctly (inhibits the carry). Do this in turn with each of the columns.

If any of the inhibit tabs bind when being pressed, you can bend them a bit to fix this but, be warned, they have a rather complex shape and you can end up with one tab looking out of place unless you're very careful.

A final personal test

Add up any list of items which already have a total (a supermarket receipt or other sale slip) and check the total. Note: avoid any subtractions at this point.

That's really about all the testing you need do to be sure your Comptometer is working properly. For those with sufficient interest, an optional set of special tests can be performed as a final guarantee that all is in perfect order.

If you are going to be actually using it very much, you should consider applying a small drop of light machine oil to the keystems, front and back. After oiling, you can wipe up any excess oil that has dripped down to the keyplate with a soft rag wrapped around a long flat stick about 1/2" wide (an emory board does nicely).

And, of course, you'll want to know how to subtract and multiply. Division, percentages, and even square root can be calculated on this "machine for all ages" if you know The Methods